Question for psychiatrists?
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Co-morbidity is common in psychiatric diagnosis and the most likely explanation is that similar mechanisms are involved in many of the disorders, hence the efficacy of SSRI's in the treatment of depression and anxiety (as well as others) and the mood stabilizing properties of anti-psychotics and so on. Diagnostic categories overlap as well and share many symptoms in common. A good example is DID where it is not uncommon for an individual with this disorder to meet diagnostic criteria for mood disorders, anxiety disorders, somatoform disorders and even psychotic disorders. Generally there is a principle of parsimony applied to diagnosis and we try to use only as many diagnoses as are necessary to fully accomodate all the symptoms. Thus an individual with DID would not be diagnosed with all of the other possible disorders that they meet criteria for as almost all symptoms are subsumed by the DID diagnosis, though occasionally another diagnosis may be required additionally to accurately reflect symptoms that are present but not associated with DID (Like OCD, for example).
Trauma is another common feature in the development of a whole host of conditions and can lead to multiple symptoms within any given individual which necessitates multiple diagnoses.
Few disorders "evolve" into others with the exception of a few. An example would be Schizophreniform Disorder which may evolve into Schizophrenia once the duration criteria are met. Another example might be an NOS diagnosis that becomes clearer or more defined over time, though usually it is a lack of clear history or relevant information which leads to an NOS diagnosis in the first place. The most common example is that of a Major Depression superimposed on a Dysthymic Disorder-the Dysthymic Disorder is generally present continuously whereas the Major Depression is recurrent and episodic, but can be viewed as a worsening of the original disorder (Dysthymia). Another common one is Conduct Disorder (diagnosed only in children/adolescents) which tends to become Antisocial Personality in males and Borderline Personality in females if left untreated. (Although women can become Antisocial, men can become Borderline, but I am speaking in terms of norms as culture and societal values certainly can influence how certain symptoms become expressed. When you consider that we are talking primarily about behaviors resulting from a combination of chemical, genetic and traumatic environmental factors, culture plays a significant role in how those factors are expressed).
There are some disorders that seem so closely related that it is often difficult to tell which came first-did chronic anxiety lead to depression or vice versa-only a clear history will usually provide adequate info as most people present with a combination of the two.
In short, many things are possible, but generally mild diagnoses do not actually evolve into greater ones, though symptoms may mutate over time. People often have histories of multiple different diagnoses at different times, but that may be a function of poor differential assessment to begin with. Adjustment Disorders don't usually become Psychotic Disorders and so forth.
Hope this helps to clarify things a bit as it is a complicated question that you asked! I'm not a psychiatrist, but I've had plenty of experience with diagnosis and I present workshops on differential diagnosis to our clinical staff as part of my job.
There is nothing to prevent you from first developing one and then another disorder - psychiatric or otherwise - without a special reason; just bad luck.
It is indeed possible for people with depression to subsequently develop symptoms of schizophrenia, too; the delineation of both diseases is not very sharp.
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